1. Field
Embodiments of the present invention relate to barcode scanners and, in particular, to signal conditioning in barcode scanners.
2. Discussion of Related Art
Barcodes have many uses including identifying consumer goods. Merchants affix barcodes to the goods and at checkout, for example, and the barcode is scanned to reveal the price of the particular goods. FIG. 1 is a high-level block diagram of a barcode-scanning platform 100. The platform 100 includes a barcode 102 that is scanned using a scanner 104 that emits an optical signal 105. A modulated optical signal 107 is reflected off the barcode 102 and a photodetector and input stage 106 converts the modulated optical signal 107 to an analog signal 108 representative of the barcode 102. A signal conditioner 110 processes the analog signal 108 and generates a digital signal 112 representative of the barcode 102.
The example barcode 102 includes series of bars 120 and spaces 122 of different contrast and widths. The particular placement and width of the bars 120 and spaces 122 form a code, which can be decoded to provide meaningful information to a user of the platform 100.
In the illustrated embodiment, the bars 120 are darker than the spaces 122. Because the bars 120 are darker than the spaces 122, the bars 120 are more absorptive and less reflective than the spaces 122. As a result, the bars 120 produce negative peaks in the analog signal 108 and the spaces produce positive peaks in the analog signal 108.
The optical signal 105 is generally a focused optical “spot” that scans the barcode, and the size of the spot affects the reading of the barcode 102. For example, if the optical spot size is smaller than the smallest bar 120 or space 122, then the optical signal 105 is said to be within optical “focus” and the analog signal 108 can be a fairly good representation of the barcode 102. One characteristic of a “focused” signal is the positive and negative peaks in the analog signal 108 for the narrowest bars 120A and spaces 122B are the same amplitude as for the wider bars 120C and spaces 122C.
As the scanning laser 104 is moved away from optical focus (e.g. farther from or nearer to the barcode 102) the optical spot size becomes larger. With the larger optical spot, the energy in the optical signal 105 is distributed across adjacent bars 120 and spaces 122 so that no one particular bar 120 absorbs all the available energy or no one particular space reflects all the available energy. As a result, the negative and positive peaks for the narrowest bars (e.g., 120A and 120D) and narrowest spaces (e.g., 122B) have less amplitude than for the wider bars 122C and spaces 122C.
One characteristic of barcode scanning platforms and the focus point is that sometimes the amplitudes of the portions of the analog signal 108 associated with the narrow bars and spaces are different from (e.g., less than) the amplitudes of the portions of the analog signal 108 associated with the wider bars and spaces. This commonly occurs when the optical spot size is larger than the narrowest bars and spaces. The ratio of amplitudes of the portions of the analog signal 108 associated with the narrow bars and spaces to the amplitudes of the portions of the analog signal 108 associated with the wide bars and spaces is referred to as a “modulation transfer function” or MTF and it can be used to describe the characteristics of the analog signal 108. For example, when the MTF is less than one hundred percent (100%), it can be difficult for the analog signal 108 to be accurately resolved into a digital signal representative of the barcode 102.
Another characteristic of barcode scanning platforms is that as the optical spot size becomes so small (e.g., at the focus point), the energy in the optical signal 105 is more concentrated on the material carrying the barcode 102 (e.g., paper fibers, wood shavings, metal pits and grains, etc.). The fibers, grains, pits, etc. themselves begin to absorb and/or reflect the energy in the optical signal 105 and cause what is commonly referred to as “paper noise.” Paper noise tends to degrade the signal-to-noise ratio (SNR) of the analog signal 108. Other noises can also be introduced in the platform. When the SNR of the analog signal 108 is degraded, it can be difficult for the analog signal 108 to be accurately resolved into a digital signal representative of the barcode 102.
Still another characteristic of barcode-scanning platforms is that the analog signal 108 should maintain constant amplitude in order to be accurately resolved into a digital signal representative of the barcode 102. Automatic gain control circuits are traditionally used to maintain the amplitude constant. However, many known automatic gain control techniques require complex control equations and high cost circuits.